This application claims the benefit of Application No. 09-94742, filed in Japan on Mar. 28, 1997, which is hereby incorporated by reference.
1. Field of the Invention
This invention relates to a production method of a distributed feedback (DFB) semiconductor laser element and a distributed feedback semiconductor laser produced thereby.
2. Description of the Prior Art
The distributed feedback semiconductor laser has been conveniently used in an optical communication system, such as an optical CATV, as a short-wave laser, which uses a second harmonic generation (SHG) element, a pump light source for a small solid laser, an optical measurement field, and similar applications. The conventional distributed feedback semiconductor laser element is formed by a so-called 2-stage epitaxial growth. For the 2-stage epitaxial growth, grating is provided on a waveguide layer of a laser element, then other layers are formed on the waveguide path by epitaxial growth.
FIGS. 4A-4D and 5A-5C illustrate production processes for the conventional distributed feedback semiconductor laser element. FIGS. 5A-5C illustrate post processes of FIGS. 4A-D. In FIG. 4A, on an InP substrate 301, n-InP lower cladding layer 302 and grating supplying layer 303a are formed by a first crystal growth by a predetermined epitaxial growth method, such as a liquid-phase growth method, an organic metal vapor-phase growth method, a molecular beam growth method, or similar process.
As shown in FIG. 4B, the grating layer 303a from FIG. 4A is then exposed by beam interference, so that a grating layer 303b is formed in a grating forming process. As shown in FIG. 4C, an active layer 304, in which the formed grating is buried, a p-InP upper cladding layer 305 and a contact layer 306 are then formed by a second crystal growth, i.e., a regrowth process, on the grating layer 303b. 
As shown in FIG. 4D, a ridge-forming mask (not shown) is formed on the contact layer 306. The contact layer 306 and cladding layer 305 are then etched so that ridge 307, having flat side portions 306 and a flat top face protruding therefrom at a predetermined height, is formed.
As shown in FIG. 5A, after the processes depicted in FIGS. 4A-4D are completed, an insulation film 309, such as a spin on glass (SOG) film, a polyimide film or the like is formed over the flat side portions 308 and the flat top face of the ridge 307.
As shown in FIG. 5B, the insulation film 309, on the contact layer 306, is then removed by etching until the contact layer 306 is reached. As shown in FIG. 5C, a metal conductive layer made of Au/Zn or the like is then formed by vacuum deposition or a similar method as electrode 310, so that a ridge stripe type DFB semiconductor laser element is formed. An electrode is also formed on the substrate 301 opposing the electrode 310.
To reduce the complexity required in producing a 2-stage epitaxial growth a so-called non-regrowth distributed feedback semiconductor laser element has been developed. Such laser element is formed on a flat substrate by a 1-stage epitaxial growth method and does not undergo a re-epitaxial growth.
For example, distributed feedback semiconductor laser elements having only a refractive index coupling have been developed, in which the active layer, cladding layer, or similar layer are formed on a substrate by epitaxial growth so as to form a ridge portion. A grating is provided on the top face and flat side portions of the ridge portion.
In the production method of the non-regrowth distributed feedback semiconductor laser element the grating is formed by an electron beam drawing method, a synchrotron radiating X-ray (SOR-X-ray) drawing method, or similar method.
However, in the production method of the aforementioned conventional distributed feedback semiconductor laser element, after grating is formed each stripe-shaped grating must be formed separately by one of the drawing methods. Thus, the amount of time to produce the grating, and hence the laser element, is increased. Furthermore the production apparatus is expensive, thereby increasing production costs.
The non-regrowth method suffers from an additional problem in that the beam interference exposure method (2-beam interference) cannot form the grating uniformly near a ridge because of beam diffraction.
Accordingly, the present invention is directed to a distributed feedback semiconductor laser element and a method for producing the same that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.
The present invention has been proposed to solve the problems enumerated above, and it therefore is an object of the invention to provide a production method of a distributed feedback semiconductor laser element which can be produced easily and simply.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, a method for producing a distributed feedback semiconductor laser element includes the steps of forming a laser substrate, forming a ridge by etching the laser substrate, forming a flattening layer on the ridge, forming a grating on the flattening layer, transferring the grating to the laser substrate on which the ridge is formed, removing the flattening layer, and forming electrodes.
In another aspect, the distributed feedback semiconductor laser element includes a laser substrate, wherein a ridge is formed by etching the laser substrate, a flattening layer, which is formed on the ridge of the laser substrate, a grating, which is formed on the flattening layer and wherein the grating is transferred to the laser substrate on which the ridge is formed and the flattening layer is removed, and electrodes.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.